The present invention relates to an improvement in a magnetic recording medium or, more particularly, to an improvement in a magnetic recording medium of the type consisting of a silicon-based non-magnetic substrate plate and a magnetic recording layer of a magnetic alloy formed thereon by sputtering or other dry process and having usefulness as an information signal-recording medium in electronic data processing.
While magnetic recording media of the above mentioned type play a core role as an external memory device in computers, they are required to have a recording capacity and recording density increasing year by year and extensive investigations therefor are now under way throughout the world in order to obtain a still higher recording density. As is known, the recording density is not determined by the characteristics of the recording medium alone but a factor having influences on the recording density of a magnetic recording medium is the so-called flying height of the magnetic head, i.e. the distance between the surface of the magnetic recording medium and the magnetic head for writing the signals of information in and reading the recorded signals out of the magnetic recording medium as held above the surface of the rotating magnetic recording medium with a very narrow gap. Namely, the recording density in a system of a magnetic recording device consisting of a magnetic recording medium and a magnetic head is increased by decreasing the flying height of the magnetic head. As is readily understood, the flying height of the magnetic head cannot be decreased without increasing the flatness and/or smoothness of the surface of the recording medium which in turn is mostly determined by the flatness and smoothness of the substrate plate on which the magnetic recording layer is formed to complete a magnetic recording medium.
Along with the trend in the modern computer technology toward more and more compact computers such as the so-called note-book-type and palm top-type computers, the magnetic recording device is also required to have a thickness as small as possible so that the magnetic recording medium is also required to have a decreased thickness. Since the principal fraction of the thickness of a magnetic recording medium is occupied by the thickness of the non-magnetic substrate plate, an important problem therefor is to develop a substrate plate which is highly resistant against distortion or deformation by an external force even when the thickness thereof is greatly decreased to comply with the above mentioned requirement.
The most conventional material of the substrate plate of a magnetic recording medium in the prior art is an aluminum alloy and it is usual that a substrate plate of an aluminum alloy is provided with an undercoating layer of a nickel-phosphorus alloy. Since aluminum alloys in general are soft as a metallic material, it is a difficult matter to obtain full flatness and smoothness of the surface of a substrate plate by grinding and distortion or warping sometimes occurs in a substrate plate of an aluminum alloy during mechanical working, especially, when the thickness of the substrate plate is small. In this regard, non-magnetic materials having a larger hardness than aluminum alloys have been proposed and practically under use as an alternative material of aluminum alloys including glass, ceramics and single crystal silicon wafers used in the manufacture of semiconductor devices (see, for example, Japanese Patent Publication No. 2-41089 and Japanese Patent Kokai No. 57-105826), of which silicon is preferred.
As to the relative positions of a magnetic recording medium and a magnetic head in a magnetic recording device, on the other hand, it is usual that, when the magnetic recording medium is rotating, the magnetic head is not in direct contact with the surface of the magnetic recording medium but is floating above the surface by means of the lift due to the air flow caused by the rotation of the medium by keeping a so-called flying height therebetween but, when the magnetic recording medium is stationary, the magnetic head rests on the magnetic recording medium in direct contact therewith. This mode of the relationship between the magnet-ic recording medium and the magnetic head is referred to as the CSS (contact-start-stop) mode and the area of the surface of the recording medium on which the magnetic head rests when in stationary state is called the CSS zone, which is usually near to the inner periphery of the magnetic recording medium in an annular form. The requirement for flatness and smoothness of the magnetic recording medium cannot be free from the problem inherent in the above mentioned CSS mode of the magnetic recording device because, when the surface of the magnetic recording medium is too flat and smooth, a resistive force like attraction called stiction is generated against starting of the magnetic recording medium into rotation from the stationary state to greatly affect the so-called CSS characteristic of the system. This situation just counteracts against the requirement for the increased recording density by increasing the flatness and smoothness of the surface of the magnetic recording medium.
In this connection, proposals have been made in Japanese Patent Kokai No. 5-258293, No. 5-205258, No. 5-166176, N5-151562 and No. 5-094617 for a method of texturing of the substrate surface, for example, by gently rubbing the substrate surface with an abrasive tape in one or more specified directions to form ruggedness on the surface (see Japanese Patent Kokai No. 4-259908). This method is based on the fact that the attractive force or stiction between the magnetic head and the magnetic recording medium against starting is proportional to the true contacting area between the magnetic head and the magnetic recording medium so that it is expected that the true contacting area can be decreased by texturing or roughening of the substrate surface. No quite satisfactory results have been obtained, however, by these methods because it is an extremely difficult matter to obtain a good balance between the incompatible requirement for the increased recording density and the requirement for the improvement of the CSS characteristic if not to mention the disadvantage in this abrasive method that deep scratches are sometimes formed in the substrate plate to greatly decrease the mechanical strengths thereof.
Japanese Patent Publication No. 2-59523 discloses a method for the texturing or surface roughening applicable exclusively to a substrate plate of silicon in which protrusions of an abrasive material are provided to protrude on the substrate surface but this method is unpractically complicated involving the steps of doping of the silicon surface with boron, formation of an aluminum layer thereon, annealing and so on. In addition, the height of the protrusions cannot be well controlled so that the height sometimes reaches 1 .mu.m or even larger to exceed the thickness of the magnetic recording layer to be formed thereon so that the flying height of the magnetic head cannot be small enough.